A VEHICLE ANTI-COLLISION DEVICE

Abstract: A vehicle anti-collision device (10) comprising a laser range finder (14, 122) mounted inside a vehicle (25, 131) near to a windshield (26, 143) thereof. The distance of the vehicle (25, 131) from a leading vehicle (29, 130) is constantly calculated and sampled at predetermined time intervals, enabling the closing velocity of the vehicle with respect to the leading vehicle to be calculated. A digital speedometer (15) is provided for determining the self-speed of the vehicle whereby the safety time between the two vehicles may be calculated. The safety time is displayed and an audible alarm is sounded in the event that the calculated safety time is less than a predetermined safety threshold. Preferably, the rangefinder (14, 122) is coupled to the vehicle's steering mechanism so as to be responsive to a turning radius of the vehicle in order that the rangefinder may distinguish between two vehicles travelling in the same lane and in different lanes, respectively. Likewise, provision (17) is made for compensating for the elevation angle of the rangefinder beam for variations in load distribution within the vehicle. The safety threshold of the rangefinder comprises a first low S/N ratio frequency adaptive threshold together with a second digital threshold set to eliminate false targets.

Description: A Vehicle Anti-Collision Device FIELD OF THE INVENTION This invention relates to a vehicle anti-collision device for aiding a driver in maintaining a safe distance between his vehicle and the one in front. BACKGROUND OF THE INVENTION Failure to keep the correct distance between two vehicles following one another is one of the principal factors leading to road accidents. Conforming the distance between adjacent vehicles to their speeds under variable conditions is subjective and depends on many factors. Thus, for example, lighting conditions, ambient weather, traffic density, road conditions, driver alertness and so on are all factors which can influence a driver's ability to keep a safe distance between two vehicles which, if not properly maintained, can result in a rear-end collision. Existing anti-collision systems are either passive or active. Passive devices warn the driver of a rear, or following, vehicle when he is approaching too close to the vehicle in front (hereinafter the "leading" vehicle) for his current road speed. The warning having been provided, the initiative is now the driver's to take suitable precautionary action. In contrast to this, active devices include a servo-control system typically coupled to the throttle valve so that, in the event that the driver does not maintain a safe distance from the leading vehicle for his current road speed, the following vehicle is automatically decelerated so as to reduce its road speed to a safe stopping speed based on the reduced distance between the two vehicles. U.S. Patent No. 4,706,195 (Yoshino et al.) describes such an active speed control system. The system disclosed by Yoshino et al. is based on a laser radar for measuring the distance of the following vehicle from the leading vehicle, the speed of the following vehicle being determined by a speed sensor coupled to the speedometer. It is clear that passive systems also must comprise at least these two elements: namely some sort of rangefinder for determining the distance between two vehicles and also means for determining the speed of the following vehicle. However, in practice, it has been found that both passive and active systems have proven difficult to exploit commercially for several reasons. First, it is difficult to design a system which is easily amenable to coupling to all existing vehicles without requiring special customization for each vehicle. As soon as such customization is required, this raises the price of the system. A further drawback relating to the development of such systems concerns the occurrence of false alarms. When a false alarm is obtained with active systems, the result is not merely irritating but can even be dangerous in that it is liable to lead to a rear-end collision between the following vehicle and the one behind it. This can occur because the following vehicle abruptly slows down (or stops) for no reason, leaving the vehicle behind him insufficient time to take precautionary action and thereby leading to a collision between the two vehicles. In passive systems, false alarms will produce similar dangerous results if the driver panics as a result of the alarm, applying his brakes too abruptly and resulting possibly in skidding of his own vehicle or, for the same reasons as explained above, a rear-end collision between his vehicle and the one behind. On a straight, level road with no camber and a uniformly distributed load in the following vehicle, it is relatively easy to design laser rangefmders which produce reliable and accurate results. However, in practice, such ideal conditions are rarely met. A non-uniform load distribution in the following vehicle can sufficiently alter the angle of inclination of the laser beam to produce erroneous results even on a level road surface. Thus, for example, people sitting on the rear seat or the addition of heavy luggage in the trunk of the vehicle can produce incorrect results in prior art systems. Likewise, as the following vehicle adjusts the steering angle in order to turn a curve, the direction of the laser beam, which remains coaxial with the longitudinal axis of the following vehicle, can miss a leading vehicle in the same lane or can strike a vehicle in an adjacent lane. Yet a further consideration is how the speed of the following vehicle is determined. In vehicles having digital speedometers, it is straightforward to extract the digital readout thereof for use in a digital anti- collision system. However, many existing motor vehicles still employ analog speedometers, in which case it is necessary to extract the analog output thereof and convert it to an equivalent digital signal for use with a digital anti-collision system. As indicated above, this requires customization since not all analog speedometers are identical. Such customization increases the price of the system and may well render it commercially unacceptable. Yet a further drawback associated with existing anti-collision devices is that the rangefinder is usually mounted outside of the vehicle typically near the fender, in accordance with the rationale that this is the initial point of contact with the leading vehicle in the event of a collision. However, mounting the rangefinder outside of the vehicle renders it susceptible to ambient weather conditions, to atmospheric pollution including the exhaust gases of the leading vehicle, and indeed to dirt which settles on the rangefinder's optics, thereby causing distortions and inaccuracies. Laser rangefinders for use in vehicle anti-collision systems are well known and must address a variety of different requirements. Thus, such rangefinders must be able to detect a target vehicle driving in the same lane as a vehicle to which the rangefinder is fitted, whilst being able to distinguish from false targets. The rangefinder must be able to work reliably in adverse weather conditions and the laser must be so designed as not to represent a hearth hazard. Additionally, cost must be kept to a minimum so as to prevent the anti-collision system from being prohibitively • expensive. These requirements are met in prior art rangefinders by using a very narrow laser beam boresighted to a visual telescopic sight. The target is placed in the middle of the field of view and a high powered laser is employed for detecting the target. Such an approach is not suitable for use in an automatic system having a wide field of view wherein it is clearly essential to ensure that only reflections from the target vehicle are measured, and stray reflections from other, irrelevant objects within the field of view are ignored. One possible solution is to scan the field of view with a narrow laser beam, to collect all relevant data for digital manipulations and to find a criterion which distinguishes vehicles from other targets. Such an approach is expensive and is unreliable in fog. U.S. Patent No. 4,948,246 describes such an approach wherein a wide field of view is scanned horizontally by a narrow laser beam so as to obtain a pattern characteristic of a motorcar. The method is complicated and does not permit identification of motorcycles or other types of motor vehicle. U.S. Patent No. 4,757,450 employs a plurality of laser beams, each directed so as to cover a respective volume within the field of view whereby, in combination, a wide field of view may be covered. Appropriate calculations permit false targets to be eliminated but the resulting system is neither simple nor does it permit accurate detection in fog. In most existing vehicle anti-collision systems employing laser rangefinders, the rangefinder itself is mounted outside of the vehicle, typically on the front fender. This is not entirely satisfactory, because dirt and grime may accumulate on the rangefinder, thereby derogating from the measurement accuracy and requiring constant maintenance and cleaning in order to avoid such errors. Also, fitting the rangefinder outside the vehicle • lends it vulnerable to tampering and theft. Yet a further consideration is that since the instrumentation relating to the anti-collision system is mounted inside the vehicle, special wiring is required in order to connect the rangefinder to the instrumentation. It is clearly essential in any rangefinder system to distinguish genuine targets from false targets. In the prior art systems, this is typically achieved by using a large S/N (signal to noise) ratio so that only strong reflections are interpreted as being derived from genuine targets. Whilst this approach does eliminate false targets, it reduces the resulting sensitivity of the rangefinder so that reflections from genuine targets in adverse conditions are often not sufficiently strong and are therefore rejected as noise signals. For example, fog tends to disperse the laser beam so that by the time the laser beam has been reflected from a true target vehicle, the strength of the received signal is so weak that it is misinterpreted as noise in conventional systems and is therefore rejected. It would clearly be preferable to mount the rangefinder inside the vehicle in order to avoid all of these potential drawbacks. However, it has proven very difficult to achieve this objective without seriously compromis¬ ing the resulting accuracy of the rangefinder and without making it very much more difficult to distinguish genuine vehicles from peripheral false targets. Specifically, part of the laser beam is reflected by the windshield itself, thereby decreasing the intensity of the transmitted beam. Further- more, vehicle windshields are typically formed of laminated glass, there being provided two laminates in between which is fitted a sun-absorption layer. This also absorbs some of the energy of the laser beam, resulting in a further loss of laser power. Whilst one possible solution to the loss of laser power is simply to employ a higher power laser, this increases the cost of the laser rangefinder and reduces the "eye safety" of the instrument. Consequently, laser rangefinders generally continue to be mounted outside of a vehicle, the drawbacks notwithstanding. Yet a further consideration relates to the manner in which the device presents data to the driver. The alarm itself is, of course, typically audible. However, no less important than the alarm itself, is the calculated collision time between the two vehicles and whose rate of change is an important guide in preserving a safe stopping distance even before any alarm signal is sounded. If such data is presented to the driver via an instrument on the dashboard, this requires that the driver look down at the dashboard, thereby momentarily taking his eyes off the road. This is particularly hazardous in those situations where there exists a high probability of a collision. It is precisely in such situations that a driver must be completely aware of prevailing road conditions without, even momentarily, losing sight of the leading vehicle. SUMMARY OF THE INVENTION It is an object of the invention to provide a vehicle anti-collision device in which the drawbacks associated with hitherto proposed devices are substantially reduced or eliminated. It is a further object of the invention to provide a laser range- finder for use with a vehicle anti-collision system and which may be mounted inside a vehicle whilst nevertheless overcoming many of the drawbacks associated with hitherto-proposed systems. According to a broad aspect of the invention there is provided a vehicle anti-collision device comprising: a rangefinder for mounting inside a following vehicle near to a windshield thereof for measuring a distance of said vehicle from a leading vehicle, distance sampling means coupled to the rangefinder for sampling measured distances at predetermined time intervals, self speed determination means for measuring a self speed of the following vehicle, collision time determination means coupled to the rangefinder and to the self speed determination means and responsive to the measured self speed of the following vehicle for determining a collision time between the following and leading vehicles; comparing means coupled to the collision time determination means for comparing said collision time with a predetermined threshold, and alarm means coupled to the comparing means for generating an alarm if the collision time is less than said predetermined threshold. Preferably, the rangefinder includes a laser light source for emitting a narrow angle beam of laser light. One component of the laser beam passes through the windshield so as to be reflected by a leading vehicle, whilst a second component of the laser beam is reflected directly by the windshield back towards the device. The device is responsive to a difference in time between receipt of both components for determining the distance between the following and leading vehicles. According to a further aspect of the invention, there is provided in a laser rancefinder for fixing to a vehicle and including therein: a laser source for directing a laser beam on to a tarcet vehicle for determininε a distance between the two vehicles, means for successively directing the laser beam at the target vehicle at predetermined intervals of time so as to be reflected thereby as a reflected beam, means for receiving the reflected beam so as to determine successive distance measurements between the two vehicles, and comparing means for comparing the successive distance measurements or a derivative thereof with a respective safety threshold so as to generate an alarm signal if one or more of the distance measurements or its derivative is less than the respective safety threshold; the improvement wherein the safety threshold comprises a first low S/N ratio frequency adaptive threshold together with a second digital threshold set to eliminate false targets. In a preferred embodiment according to the invention, the self- speed of the following vehicle is determined by counting pulses of light reflected by a light-reflecting strip adhered to an axle of the following vehicle: such an approach being universally applicable and obviating the need for customization to different vehicles. In the improved rangefinder according to the invention, a low S/N ratio is employed so that weak signals, which may possibly result from true targets in adverse weather conditions such as fog, for example, are not rejected as they are in hitherto proposed systems. Since, nevertheless, such weak signals could derive from false targets, the resulting reflected signals are subjected to further analysis in order to distinguish true targets from false targets. BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how the same may be carried out in practice, a preferred example will now be described, by way of non-limiting example only, and with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing functionally the principal compo- nents of a device according to the invention; Fig. 2 shows pictorially a device according to the invention mounted inside a vehicle; Fig. 3 shows pictorially a digital speedometer for use with the device; Fig. 4 is a flow diagram showing the principal steps associated with a method for determining the self-speed of a vehicle; Figs, δa and 5b are flow diagrams showing the principal steps associated with a method for determining the speed of a leading vehicle as well as the collision time between two vehicles; Fig. 6 is a schematic diagram showing the main components in a receiver of the rangefinder shown in Fig. 1; Figs. 7a, 7b and 7c are geometric diagrams showing how the system according to the invention discriminates between vehicles in different lanes; Fig. 8 is a pictorial representation of a vehicle having height transduc¬ ers in accordance with a further embodiment of the invention; Fig. 9 is a geometric representation useful for explaining operation of the embodiment shown in Fig. 8; Fig. 10a is a schematic representation of a prior art rangefinder; Fig. 10b is a graphical representation relating to the prior art range- finder shown in Fig. 10a; Fig. 11 is a schematic representation of a rangefinder according to the invention; Fig. 12 is a graphical representation relating to the rangefinder shown in Fig. 12; Fig. 13 is a pictorial representation of a laser beam reflected from a true, moving target and a false, stationary target; Fig. 14 is an intensity versus time characteristic for the situation depicted in Fig. 13; Fig. 15 relates to Fig. 13 and shows a frequency versus range characteristic; Fig. 16 is a pictorial representation of a laser rangefinder according to the invention, fitted inside a vehicle; Fig. 17 is a schematic representation of a front windshield of the vehicle shown in Fig. 16; and Fig. 18 shows schematically a detail of the windshield shown in Fig. 16 having affixed thereto a prismatic element. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to Fig. 1 there is shown a block diagram of a device depicted generally as 10 comprising an instrument 11 powered by a 12 V power supply 12. The power supply 12 is built as a separate system, assembled underneath a dashboard of a vehicle, and supplies the different voltages, as required, to the instrument 11 for the operation of the different electronic systems therein. The instrument 11 comprises a micro-controller 13 coupled to a laser rangefinder 14, to a speed transducer 15, to a steering angle transducer 16 and to an elevation angle transducer 17. The micro-controller 13 is also coupled to a display 18 and to a buzzer 19. The display 18 and the buzzer 19 are contained within the instrument 11, whilst the speed transducer 15 and the steering angle transducer .16 are provided as separate units which are externally coupled to the micro-controller 13. The laser rangefinder 14 comprises a transmitter 20 coupled to a laser diode and suitable optics 21. The laser diode and optics 21 emits a narrow angle laser beam which is intercepted by a leading vehicle so as to be reflected thereby and detected by an optical detector 22 an output of which is fed to a receiver 23 coupled to the micro-controller 13. A sensitivity selector 24 connected to the receiver 23 permits the sensitivity of the receiver 23 to be adjusted so as to allow for varying weather conditions, such as fog, for example. Figs. 2a and 2b show pictorially the instrument 11 positioned within a vehicle 25 proximate a windshield 26 thereof and preferably glued thereto. The laser diode 21 (Fig. 1) emits a beam of laser light having a first component 27 which is reflected by the windshield 26 and a second component 28 which passes through the windshield 26 so as to be reflected by a leading vehicle 29 in front of the vehicle 25. When the first and second components 27 and 28 of the laser beam strike the optical detector 22, corresponding first and second detector signals are generated thereby and fed to a timer 30 which measures an elapsed time Δt between receipt of the first and second components 27 and 28 of the laser beam. A. distance computer 31 coupled to the timer 30 is responsive to the elapsed time Δt for determining the distance between the two vehicles. Such an arrangement obviates the need for synchronization between the transmission of a laser pulse and subsequent receipt thereof, since both the first and second components 27 and 28 of the laser beam are derived from the same beam and any flight time delay between the actual emission of the laser beam and receipt of the first component 27 thereof by the detector 22 is exactly compensated for by the transit time for the second component 28 of the laser beam to reach the windshield 26 from the detector 22 during its outgoing trajectory and to reach the detector 22 from the windshield 26 on its return trajectory. In practice, the timer 30 and the distance computer 31 are not provided by separate hardware circuits but, rather, are provided within the micro-controller 13. A safety time threshold adjustment 33 is coupled to the distance computer 31 for adjusting the safety time threshold at which an alarm is given, in order to allow for different ambient conditions or wakefulness of the driver. A frequency of the laser diode is randomly selected between predetermined lower and upper thresholds in respect of a specific instrument, the optical detector 22 being connected to a filter 32 for filtering out a received signal whose frequency differs from the selected frequency. By such means only a received signal of the correct frequency is accepted and a laser beam (of different frequency) directed towards the detector 22 by another vehicle also having a laser rangefinder fitted thereto will be filtered out, thereby reducing false alarms. Fig. 3 shows schematically the speed transducer 15 which comprises at least one light-reflecting strip 35 adhered to an appropriate drive shaft 36 of the vehicle. In a front wheel drive, the drive shaft 36 may be one of the two drive shafts on which the front wheels 37 and 38 are mounted. In a rear wheel drive, the drive shaft 36 is constituted by the common drive shaft between the gear box and the back axle (not shown). A light source 40 emits a continuous beam of light 41 towards the axle 36 in the region of the light-reflecting strip 35 so as to be reflected thereby as a reflected beam 42 which is intercepted by a detector/counter 43 which may be optically coupled to the light-reflecting strip via an optical fiber (not shown). As the axle 36 rotates, a reflected beam 42 is emitted towards the detector/counter 43 each time the light-reflecting strip 35 intercepts the beam 41. The detector/counter 43 produces a pulse each time a reflected beam 42 is detected, each of the received pulses being counted. The number of pulses counted in a given time, in conjunction with the number of light- reflecting strips 35 provided on the axle 36, permits the number of revolutions per unit time of the axle 36 to be computed. This having been done, it is a simple matter to calculate the self-speed of the vehicle by multiplying the angular speed of the axle 36 by the radius of the wheels 37 and 38 mounted thereon. In vehicles having digital speedometers provided as standard, the digital output thereof may be fed directly to the micro-controller 13 without the need for the components shown in Fig. 3 to be provided as separate elements. However, the arrangement shown in Fig. 3 is capable of universal application and does not require special customization for different vehicles, apart from specifying the wheel radius which is the only variable in such a system. Fig. 4 is a flow diagram showing the principal steps associated with the calculation of self-speed using the arrangement described above with reference to Fig. 3 of the drawings. Thus, the system is dormant until a first pulse is received, whereupon a clock is activated, such that on receipt of subsequent pulses, the elapsed time Δt between successive pulses is measured. A correction factor is applied, as required, whereupon the self-speed of the vehicle is calculated in accordance with the equation: V = _- .- Δf where: k is a constant which takes into account the number of light- reflecting strips 35 and the wheel radius, and Δt' is the corrected elapsed time between pulses. Figs. 5a and 5b show the principal steps associated with the calculation of the speed V, of the leading vehicle and the collision time t col between the two vehicles. When the first component of the laser beam strikes the optical detector 22, a pulse is produced which activates a clock associated with the micro-controller 13. A second pulse is produced when the second component of the laser beam strikes the detector 22, thereby enabling the clock to measure the elapsed time Δt between the two successive pulses. Since the speed C of the laser beam is known, the distance travelled thereby during an elapsed time Δt may easily be calculat¬ ed. In practice, this procedure is repeated continually and the results during a given time are averaged in order to reduce inaccuracies. Denoting: S n as the calculated inter-vehicle distance during the nth iteration, and / as the pulse frequency of the laser diode, it may be shown that: In the above equation ΔV, represents the closing velocity with which the following vehicle approaches the leading vehicle. Since the self- speed of the following vehicle is known, it is therefore a simple matter to calculate the actual road speed of the leading vehicle. Additionally, since the distance between the two vehicles has been determined, the collision time between the two vehicles may also be calculated. In this connection it should be noted that, strictly speaking, the collision time is equal to the distance divided by the closing velocity, ΔVj. However, in practice, most traffic laws impose a stricter criterion for the calculation of "safety time" between two vehicles, requiring that, the distance between the two vehicles be sufficient to enable the driver of the following vehicle to react properly even in the event that the leading vehicle stops instantaneously. In this case, of course, the closing speed is equal to the self-speed of the following vehicle. The safety time having thus been calculated, it is displayed and also compared with a threshold, a buzzer being sounded in the event that the calculated safety time is less than the threshold. Fig. 6 shows the principal circuit components in the rangefinder 14 shown functionally in Fig. 1 of the drawings. A silicon PIN diode detector 45 is connected to the junction 46 of a capacitor 47 connected in series with a resistor 48 and an inductance 49, a common terminal of which 50 is connected to ground GND via a capacitor 51. A voltage of 75 V is fed to a free terminal of the inductance 49 via a positive supply rail 52 connected to the positive terminal of a zener diode 53 whose negative terminal is connected to GND. The positive supply rail 52 is connected via a pull-down resistance 55 to a positive voltage supply of 200 V. An output 56 of the laser diode 45 is connected to an input of a trans-conductance pre-amplifier shown functionally as 57, an output 58 of which is coupled via a capacitor 59 and a resistor 60 to an input 61 of a post-amplifier shown functionally as 62. The resistor 60 is connected across the input 61 and GND. An output 63 of the post-amplifier is connected to a first input 64 of a fast comparator 65 a second input 66 of which is connected to a voltage reference V rsf connected across a first resistor 67 in series with a capacitor 68, a low voltage terminal of which is connected to GND. A switch 69 in series with the capacitor 68 and the first resistor 67 permits connection of a second resistor 67' to the voltage reference V ref instead of the first resistor 67. Coupled to an output 70 of the comparator 65 is a switch shown functionally as 72 having an output thereof connected between a positive supply rail V, via a coil 75 and GND via a capacitor 76. An analog-to-digital (A D) converter 78 having an output 79 is connected across the capacitor 76. The operation of the circuit is as follows. An incoming laser pulse is detected by the PIN diode detector 45, an output of which is amplified by the trans-conductance pre-amplifier 57 in tandem with the post-amplifier 62. The output of the post-amplifier 63 is compared with V ref so that the fast comparator 65 produces an output only if the voltage appearing across its first input 64 exceeds the value of V ref . The capacitor 68 which is fully charged sets a high threshold level for the incoming pulses, so that only very strong reflections from nearby targets are detected, thereby avoiding a major cause of false alarms. As time passes, the capacitor 68 discharges through the first resistor 67 and the threshold level V ref is lowered accordingly. At the same time, the capacitor 70 is charged through the coil 75 at a constant current. When a threshold crossing is detected by the threshold detector 72, charging of the capacitor 76 stops and a voltage measurement by the A/D converter 78 is performed. The voltage appearing across the output 79 of the A/D converter 78 is proportional to the elapsed time Δt between receipt of successive pulses by the PIN diode detector 45 and therefore to the distance between adjacent vehicles. . The comparator 65 together with the switch 72 constitute a threshold detector for adjusting the sensitivity of the rangefinder in accordance with the time constant of the capacitor 68 in series with the selected resistor 67 or 67'. Thus, by selecting suitable values for the first and second resistors 67 and 67', respectively, the sensitivity of the rangefinder receiver may be increased or decreased. This is particularly useful for increasing the sensitivity of the device in adverse weather conditions, such as fog, when visibility is low so that detection of weak reflections over small distances may be effected. The coil 75 together with the capacitor 76 and the A D Converter 78 form an accurate time counter for measuring the range, this being proportional to elapsed time between receipt of successive pulses. In the particular configuration shown schematically in Fig. 2a of the drawings, successive pulses are, in fact, derived from first and second components of the beam being reflected by the windshield 26 of the following vehicle and by the leading vehicle, respectively. In this case, the elapsed time must be measured between receipt of only that component which is reflected by the leading vehicle. This requires two threshold detectors to be employed to start charging after receiving the first component pulse reflected from the windshield of the following car, and to stop charging upon receipt of the second component pulse reflected by the leading vehicle, respectively. As has been mentioned above, one of the major sources of error in hitherto proposed anti-collision systems is the occurrence of false alarms. False alarms are typically caused by the rangefinder emitting a beam which is reflected not by an adjacent vehicle but, instead, by some extraneous object such as, for example, a lamp post, a tree or even another vehicle which does not represent a danger to the following vehicle. Use of a laser rangefinder goes a long way to eliminating extraneous reflections by false objects since the dispersion angle of the laser beam is very small. However, there still exists the possible cause of error that, when turning a corner, the laser beam is directed to a vehicle in an adjacent lane rather than in the same lane as the following vehicle. Fig. 7a shows geometrically a vehicle 80 travelling along an inner lane 81 of a two lane highway depicted generally as 82 and having an outside lane 83, the outer wheels of the vehicle 80 lying on an inside edge 84 of the inner lane 81. The inner and outer lanes 81 and 83 meet at a common boundary 85 and have identical widths W. In the following analysis it is assumed that the vehicle 80 is turning a circular curve of radius R and center 0. Thus, assuming: α is the dispersion angle of the laser beam B is the point of intersection by a centra] portion of the laser beam with the adjacent lane 83 A is the point of intersection by the nearest part of the laser beam with adjacent lane 83 C is the point of intersection of the far portion of the laser beam with the adjacent lane 83 W is the width of each lane m is the width of the vehicle 80 R is the turning radius, and DC is the maximum distance travelled by the laser beam when reflected by a vehicle in the same lane 81 as the vehicle 80, then, considering Δ DBO it can be shown that: DB = J ( R + Wf - ( R + Z \ 2 BB ' = DB tan -≥- 2 Referring to Fig. 7b, in Δ ODC, sides OD and OC are known as is angle ODC. Hence side DC may be calculated using the cosine rule, as follows: OC 2 = OD 2 + CD 2 - 2 OD . CD cos (90 - - ( R + Wf = R 2 + CD 2 - 2 R. CD s ± giving a quadratic equation which can be solved for CD. Thus, there exists a maximum distance propagated by the laser beam whilst still striking a vehicle in the same lane and which varies in accordance with the dispersion angle a of the laser beam, the width W of the lane, the width m of the vehicle and the radius R of the turning circle. In practice, the dispersion angle of the laser beam, the width W of the lane and the width m of the vehicle are all constant for a particular device since road lanes are generally of constant, known width. The values of may easily be measured and stored within a memory (not shown) associated with the micro-controller 13 (Fig. 1), during manufacture of the device, whilst the values of W and m may likewise be stored upon installing the device in a vehicle for the first time. The maximum distance DC is thus a function of several stored parameters and the turning radius R and constitutes a threshold, which if exceeded by a measured propagation distance, must be rejected. Fig. 7c shows that the turning radius R is itself a function of the wheel base between the front and rear axles of the vehicle and the average turning angle of the front wheels which may be determined by measuring an offset angle of the steering mechanism relative to a base value when the vehicle is moving straight. Thus assuming: H is the wheel base of the vehicle, and θ is the average turning angle of the front wheels, then it may be shown that: R = H 4 f tan 2 6 4 tan 2 θ It will be appreciated that the above analysis is simplified and that if the vehicle 80 is closer to the boundary 85 between the inner lane 81 and the outer lane 83, the beam will travel a correspondingly shorter distance than DC prior to striking a vehicle in the outer lane 83. Thus, some false alarms may still occur, albeit many fewer than would otherwise be produced. However, more importantly, the distance propagated by the beam before being reflected by another vehicle in the inner lane 81 (i.e. the same lane) will always be smaller than the stored threshold DC and will therefore not be rejected. Fig. 8 shows pictorially a vehicle 90 having a front fender 91 and a rear fender 92. Positioned on the front and rear fenders 91 and 92 are respective transducers 93 and 94 which are at heights h } and h 2> respectively, above a road surface 95. Fig. 9 relates to the situation shown in Fig. 8, and represents geometrically the effect of varying a load distribution within the vehicle 90 such that the heights h } and h 2 vary accordingly. In the initial situation depicted in Fig. 8, there exists an angle a between the road surface 95 and an imaginary line joining the two transducers 93 and 94, given by: fl - h ) tan α = — - — L Consequent to an increased loading towards a rear end of the vehicle 90, the value of h 2 decreases giving rise to a new value a' between the road surface 95 and the imaginary line joining the two transducers 93 and 94. The variation in the angle is computed in response to the height measurements provided by the transducers 93 and 94 and is fed to a servo system (not shown) connected to the rangefinder 14, so that the angle of the laser beam emitted by the rangefinder 14 may be varied in order to compensate for variations in the angle a. It will be appreciated that modifications may be made to the device without departing from the spirit of the invention. For example, although in the embodiment described with reference to Fig. 1 of the drawings, the power supply is provided as a separate unit, it may equally well be incorporated within the device itself. Similarly other modifications will be apparent to those skilled in the art. There will now be described an improved laser rangefinder for use with the device described in detail above. It will first be useful to explain the problem to whose solution the improved rangefinder is addressed and to describe a typical prior art approach. Fig. 10a shows a typical prior art rangefinder discrimination system 100 wherein an input signal 111 is fed via a delay 112 and an attenuation 113 to respective positive and negative inputs of a first compara¬ tor 114 and to the positive input of a second comparator 115. The negative input of the second comparator 115 is fed lo a variable voltage input derived by means of a variable resistance 116 connected at one end to a voltage supply V and at its other end to GND. Respective outputs of the first and second comparators 114 and 115 are ANDed together by means of an AND gate 117, an output 118 of the AND gate 117 being logic "1" for a true target and logic "0" for a false target. Referring to Fig. 10b, the operation of the system 100 will now be explained. The attenuation 113 compensates for the inevitable attenuation associated with the delay 112. The value of the delay 112 is so arranged that the attenuated signal 119a and the delayed signal 119b cross at half the peak value of the original pulse, the crossover point constituting a "Constant Fraction Threshold" whereby the first comparator 114 operates at only a constant fraction, in this case half, of the original pulse. The variable resistance 116 is adjusted so as to provide a high signal-to-noise (S/N) ratio of between approximately 4 and 5 so as to permit the first comparator 114 to eliminate false targets. However, in the process some genuine targets are likely to be eliminated also. In such a laser ran g efinder for measuring a time difference Δt between a laser pulse and its reflection from a vehicle (or other target), the start time corresponds to the instant of transmitting the laser pulse and is determinate. The stop time corresponds to the instant that the input signal 111 is received and is not immediately determinate. This is because the input signal 111 has a non-zero rise time such that it cannot be guaranteed that measurement will commence from the zero crossing of the input signal 111 unless a zero threshold is employed. This is not practical in a real system because noise signals would be passed as bona fide signals and therefore a non-zero, positive threshold is mandatory. The problem which must therefore be solved is to establish a threshold which ensures that the stop time associated with the input signal 111 is independent of its rise time. Fig. 11 shows schematically a rangefinder system 122 in accor¬ dance with the invention, for comparing an input signal 123 with a reference threshold in order to determine whether the input signal 123 represents a true or false target. The reference threshold is derived from the input signal 123 itself which is fed through a series combination of a Low Pass Filter 124 and an attenuation means 125 and fed to the negative input of a comparator 126. The input signal 123 is fed to the positive input of the comparator 126 via a delay 127. Referring to Fig. 12, the operation of the system 100 will now be explained. The Low Pass Filter 124 subjects the input signal 123 which is SUBSTITUTE SHEET (RULE 2 β ) fed to the inverting input of the comparator 126 to an inevitable delay. The delay 127 merely ensures that the signal fed to the non-inverting input of the comparator 126 is subjected to the same delay, so as thereby to cancel the undesirable effect of the delay caused by the Low Pass Filter 124. The attenuation 125 together with the delay 127 compensate for the inevitable attenuation associated with the delay 127. The input signal 123 and the delayed signal 128 are of equal amplitude and their respective peaks are separated in time by an amount dependent on the value of the delay 127. The delayed signal 128 is fed to the non-inverting input of the comparator 126 whilst to the inverting input thereof is fed a signal 129 being the attenuated output of the Low Pass Filter 124. The signal 129 constitutes the threshold of the system 122 and, unlike the constant fraction threshold associated with Figs. 10a and 10b, varies according to the amplitude of the input signal 123. The system 122 thus constitutes an adaptive, frequency-based threshold detector. Fig. 13 shows pictorially a target vehicle 130 moving in front of . a vehicle 131 inside of which is fitted a laser rangefinder according to the invention. Also shown is a side wall 132 constituting a false target, reflections from which must be eliminated. In such a situation, the target vehicle 130 travels in a direction which is substantially normal to the vehicle 131 so that a laser beam 133 directed towards the target vehicle 130 strikes an area of the target vehicle 130 so as to be reflected by the area substantially simultaneously. By such means, the reflected beam may be construed as a plurality of component reflections, all of which reach the vehicle 131 substantially at the same time. In contrast to this, the laser beam, shown as 133 strikes an area of the wall 132 at a non-normal angle, such that, again considering the laser beam 133 to be composed of a plurality of component beams, different ones of the component beams are reflected from the wall 132 at different instances of time. Consequently, the rangefinder (not shown) situated within the vehicle 131 receives a plurality of reflected pulses, spread out in time, each representing a different distance from the vehicle 131. This permits differentiation between the moving vehicle 130 and the stationary wall 132 since a wide distribution of ranges implies that the target is a stationary distributed target, such as a wall at the side of the road, and so on. Reflections from such targets are rejected. Fig. 14 shows an intensity versus time characteristic for the reflected pulse from the vehicle 130 and the wall 132. The intensity time characteristic for the laser beam reflected from the vehicle is shown as 135 whilst that for the laser beam reflected from the wall 132 is shown as 136 It will be seen that the pulse reflected from the vehicle 130 is a relatively sharp, high intensity spike whilst the reflection 136 from the wall has a low median intensity and is somewhat spread out in time. If a threshold is now imposed on the reflected signal and it is passed through a low pass filter, the signal returning from the wall will virtually be passed in its entirety, whilst that reflected from the vehicle 130 will be significantly blocked. Since the reflected signal from the stationary, false target is passed substantially in its entirety, subtracting the filter input from the filter output effectively eliminates completely the signal from the stationary, false target, whilst allowing the signal from the moving, i.e. true, target to be passed almost in its entirety. This technique, whereby the filter output is subtracted from the filter input, is termed a "frequency-adaptive threshold", because the threshold varies in accordance with the environmen- tal conditions according to the target frequency. In practice, both the signal and the threshold contain unavoidable electronic noise and there is therefore white noise superimposed on the difference signal. In order to ensure that the noise spikes themselves do not surpass the threshold, different amplification factors are required for both channels. The low pass filter subjects the electronic signal to a delay and so, in order to conform the time scales of the two channels, a like delay is imposed in the signal channel as shown in Fig. 11. Fig. 15 shows a frequency range distribution for successively measured ranges relating to the moving vehicle 130 and the stationary wall 132. Since, in practice, both of the vehicles 130 and 131 are moving, over a relatively short period of time the inter-vehicle separation remains relatively constant, at least compared to the distance between the vehicle 131 and a stationary target, for which, of course, the mutual separation is constantly changing. Consequently, a spike 137 in the frequency-range characteristic denotes a moving, true target whose distance from the vehicle 131 is D. On the other hand, the spread out characteristic 138 denotes a stationary target whose distance from the vehicle 131 is constantly changing. This permits differentiation between a moving and stationary target. Consider, for example, that in a certain time period 200 ranges are measured. Assume, further, that at least 50 of them must be within a 3 m range to be considered a valid target. This constitutes a threshold which immediately eliminates all noise pulses which are distributed randomly over a wide range area (such as the characteristic 138 shown in Fig. 15), so that they never pass the second threshold. Likewise, this reduces the likelihood of detecting a stationary target whilst the vehicle 131 is moving because of the resulting smearing effect on the range distribution. Thus, the frequency range characteristic constitutes a digital threshold for determining whether a target is true or false. It will be understood that the efficacy of the digital threshold depends on all possible valid signals being passed in the first place. This is ensured by the selection of the analog threshold, which ensures a sufficiently low S/N ratio to allow all possible valid signals to pass. Both the analog and digital thresholds may be changed so as to negate the effects of temperature-dependent noise, such as might be caused by ambient heating and sun illumination. A third threshold constituting a safety threshold is generally employed in anti-collision systems so as to provide a warning if the separating distance between two vehicle is less than the so-called "safety distance". The safety threshold may likewise be varied so as to adjust the collision safety time which is different for moving and stationary targets, since moving targets also require time for stopping. Figs. 16, 17 and 18 show a vehicle 140 having a laser rangefinder 141 fitted therein so as to direct a laser beam 142 through a front windshield 143 of the vehicle 140. The laser beam 142 is directed through a substan- tially central portion 144 which is constantly swept by a windshield wiper 145 so as to be cleaned from the outside and prevent the buildup of dirt and grime. As shown in Fig. 16, the laser beam 142 is reflected from the inside surface of the windshield 143 so as to produce a reflected component 146 and a transmitted component 147 which, in turn, is reflected by a target vehicle. Thus, the energy associated with the transmitted component 147 is less than the original energy associated with the laser beam 142 by the energy associated with the reflected component 146. It is therefore desirable to reduce as much as possible the energy associated with the reflected component 146. This is done, in accordance with one embodiment, by coating the portion 144 with an anti-reflection coating as is known in the art. Fig. 18 shows an alternative approach whereby a prism 148 is glued to the front windshield 143 using an optical glue, the laser beam 142 being directed substantially normally to a surface 149 of the prism 148 so that the incident beam 142 is directed through the prism 148 and is refracted throuch the front windshield 143 without being reflected. Preferably, the prism 148 is coated with an anti-reflection coating so as to reduce even further stray reflections and increase the power of the transmitted component of the beam. Likewise, the glue utilized for adhering the prism 148 to the front windshield 143 should have the same refractive index as the prism 148 so as to prevent total internal reflections within the prism 148. Reflections from the front surface of the prism 148 are minimized by applying an appropriate anti-reflection coating thereto. By means of the above construction, reflections from the front windshield 143 are reduced to a minimum, thereby increasing the transmit- tance of the laser beam through the front windshield 143 and permitting a relatively low power laser to be employed, commensurate with safety considerations.

Claims: CLAIMS: 1. A vehicle anti-collision device (10) comprising: a rangefinder (14, 122) for mounting inside a following vehicle (25, 131) near to a windshield (26, 143) thereof for measuring a distance of said vehicle from a leading vehicle (29), distance sampling means (13) coupled to the rangefinder (14, 122) for sampling measured distances at predetermined time intervals, self-speed determination means (15) for measuring a self-speed of the following vehicle (25, 131), safety time determination means (13) coupled to the rangefinder (14, 122) and to the self-speed determination means (15) and responsive to the measured self-speed of the following vehicle for determining a safety time between the following and leading vehicles; comparing means (13) coupled to the safety time determination means for comparing said safety time with a predetermined threshold, and alarm means (19) coupled to the comparing means (13) for generating an alarm if the safety time is less than said predetermined threshold. 2. The device according to Claim 1, wherein the rangefinder (14) includes: a laser light source (21) for emitting a beam of laser light a first component (27) of which is reflected by the windshield, a second component (28) of which passes through the windshield (26) of the following vehicle (25) so as to be reflected by the leading vehicle (29), detector means (22) for receiving said first and second components and generating respective first and second detector signals, timing means (30) coupled to the detector means (22) and responsive to the first and second detector signals for measuring an elapsed time (Δt) between the detection of the first and second components, and distance determination means (31) coupled to the timing means (30) and responsive to said measured elapsed time for determining the distance between the following and leading vehicles. 3. The device according to Claim 1, further including leading speed determination means (13) for determining a speed of the leading vehicle (29). 4. The device according to Claim 3, wherein the leading speed determination means includes: closing speed determination means (13) coupled to the distance sampling means and responsive to a difference in successive sampled distances and said predetermined time interval for determining a closing speed of the following vehicle with respect to the leading vehicle, and difference means (13) for subtracting said closing speed from the self- speed of the following vehicle. 5. The device according to Claim 3, further including a display (18) for displaying the speed of the leading vehicle. 6. The device according to Claim 1, further including a display (18) for displaying the safety time. 7. The device according to Claim 1, wherein the self-speed determination means (15) is coupled to a speedometer of the following vehicle (25). 8. The device according to Claim 1, wherein the self-speed determination means (15) includes: at least one light-reflective strip (35) on a drive shaft (36) of the following vehicle (25), a source of light (40) directed towards the drive shaft (36) so as to strike the light-reflective strip (35) during each revolution of the drive shaft, a detector (43) for receiving a reflected beam of light (42) reflected by the light-reflective strip and generating a detector signal, a clock (13), a counter (13) coupled to the clock and to the detector (43) for counting a number of detector signals in a predetermined time interval so as to determine a number of revolutions of the drive shaft in said time interval thereby permitting the self speed of the following vehicle to be calculated. 9. The device according to Claim 2, wherein the rangefinder (14) is coupled to a steering mechanism of the following vehicle (25), there being further included: turning angle determination means (16) coupled to the steering mechanism for determining a turning radius thereof, lane differentiation means (13) coupled to the turning angle determina¬ tion means and responsive to said turning radius (R), to a known dispersion (α) of said laser beam, to a known lane width (W) and to a known vehicle width (m) for determining a maximum distance (DC) between the following vehicle (25) and the leading vehicle (29) for which the two vehicles are in identical lanes, and switching means (13) coupled to the lane differentiation means, to the turning angle determination means and to the detector means for intercepting the reflected first component of the laser beam and directing it to the detector means only if the two vehicles are in identical lanes. 10. The device according to Claim 9, wherein the turning angle determination means (16) is responsive to a known wheel base (H) of the vehicle and to an offset angle (θ) of the steering mechanism relative to a predetermined angle corresponding to a straight trajectory of the vehicle. 11. The device according to Claim 2, further including an elevation adjustment means (17) coupled to the laser light source for adjusting an elevation angle (a) of the laser beam such that the elevation angle is substantially constant regardless of load distribution within the following vehicle. 12. The device according to Claim 1 , further including safety time adjustment means (33) for adjusting said predetermined threshold. 13. The device according to Claim 2, wherein a laser frequency of the laser light source is randomly selected between predetermined lower and upper thresholds in respect of a specific vehicle, and there are further provided filtering means (32) for filtering out a received signal whose frequency differs from said laser frequency, thereby preventing false alarms owing to receipt of spurious signals in a region of said specific vehicle. 14. The device according to Claim 1 , wherein the rangefinder includes a sensitivity selection means (24) for adjusting a sensitivity thereof. 15. In a laser rangefinder (14, 122) for fixing to a vehicle (25, 131) and including therein: a laser source (21) for directing a laser beam (133) on to a target vehicle (29, 130) for determining a distance between the two vehicles, means (20) for successively directing the laser beam at the target vehicle (29, 130) at predetermined intervals of time so as to be reflected thereby as a reflected beam, means (22) for receiving the reflected beam so as to determine successive distance measurements between the two vehicles, and comparing means (13) for comparing the successive distance measure¬ ments or a derivative thereof with a respective safety threshold so as to generate an alarm signal if one or more of the distance measurements or its derivative is less than the respective safety threshold; the improvement wherein the safety threshold comprises a first low S/N ratio frequency adaptive threshold together with a second digital threshold set to eliminate false targets. 16. The improvement according to Claim 15, wherein the false targets include stationary targets (132) disposed non-normally to said laser beam (133) such that an area of the stationary target is successively bombarded by and reflects the laser beam, whereby an intensity/time profile of the reflected beam is relatively spread out compared to that of a target substantially normal to the laser beam. 17. The improvement according to Claim 15, wherein the first frequency adaptive threshold is adjusted to eliminate reflections from fog and the second digital threshold is adjusted to detect low intensity reflec- tions. 18. The improvement according to Claim 16, wherein the first frequency adaptive threshold is adjusted to eliminate reflections from fog and the second digital threshold is adjusted to detect low intensity reflec¬ tions. 19. The improvement according to Claim 15, wherein the first or second threshold is adjusted so as to reduce noise influenced by ambient • conditions. 20. The improvement according to Claim 15, wherein the safety threshold is variable to allow for moving and stationary targets. 21. The improvement according to Claim 15, wherein the false targets include stationary targets whose respective distances from the rangefinder (122) vary as the vehicle moves towards or away from the stationary targets, thereby permitting the stationary targets to be distinguished from a moving vehicle in front of the rangefinder which maintains a relatively constant distance therefrom. 22. The improvement according to Claim 15, wherein the laser beam (133) is directed through a portion of a windshield (143) of said vehicle having applied thereto an anti-reflection coating for increasing a transmit- tance of the laser beam therethrough. 23. The improvement according to Claim 15, wherein the laser beam (133) is directed through a portion of a windshield (143) of said vehicle from which a sun-absorbent layer is removed and a transparent layer substituted therefor. 24. The improvement according to Claim 15, wherein the laser beam (133) is diverted through a prismatic element (148) fixed to an inner surface of a windshield (143) of said vehicle so as to reduce an incident angle between the laser beam and the windshield. 25. The improvement according to Claim 24, wherein the prismatic element (148) is coated with an anti-reflection coating. 26. The improvement according to Claim 24, wherein the prismatic element (148) is fixed to the windshield (143) by means of a glue having a refractive index equal to that of the prismatic element. 27. The improvement according to Claim 15, wherein the laser beam (133) is diverted through a portion of a windshield (143) of said vehicle within an area (144) swept by a windshield wiper of the vehicle so as to clean said portion and increase transmittance therethrough of the laser beam.

Inventors: GAVISH MOSHE, KANTOR ISRAEL, SCHECHNER PINCHAS, YAVNAYI AMOTZ, GAVISH, MOSHE, KANTOR, ISRAEL, SCHECHNER, PINCHAS, YAVNAYI, AMOTZ
Applicants: GAVISH MOSHE, KANTOR ISRAEL, SCHECHNER PINCHAS, SHACHTER ISRAEL, SILICON HEIGHTS LTD, YAVNAYI AMOTZ, GAVISH, MOSHE, KANTOR, ISRAEL, SCHECHNER, PINCHAS, SHACHTER, ISRAEL, SILICON HEIGHTS LTD., YAVNAYI, AMOTZ
Application year: 1994
Main classification: G01S17
IPC codes: B60R21, B60W30, G01S17